1. Field of the Invention
The present invention relates to a power unit for a vehicle. More particularly, the present invention relates to a power unit for a vehicle, in which a crankshaft, first and second main shafts and a single countershaft are rotatably supported by a crankcase with their axes being parallel to each other, multispeed gears to be selectively established are provided among the first main shaft, second main shaft and countershaft, a single driving gear that makes up a part of a primary reduction gear mechanism for transmitting rotary power from the crankshaft to the first and second main shafts is mounted on the crankshaft, and first and second clutches that transmit/interrupt the rotary power from the primary reduction gear mechanism to the first and second main shafts are disposed at ends of the first and second main shafts, respectively.
2. Description of the Background Art
There is a known power unit for a vehicle. An example of such power unit is disclosed in the JP-A No. 2008-303939, in which selectively established multispeed gears are provided among a pair of main shafts disposed parallel to each other and a countershaft having an axis parallel to the main shafts, and a pair of clutches that transmit/interrupt rotary power from a crankshaft to the pair of main shafts are disposed at ends of both the main shafts, respectively.
However, the power unit disclosed in the JP-A No. 2008-303939 is configured so that a driving gear, which is provided on the crankshaft coaxially disposed with the countershaft, meshes with a driven gear, which is coaxially and rotatably supported by the first and second main shafts. In other words, in the power unit as disclosed in the JP-A No. 2008-303939, the center distance between the first and second main shafts is determined by a diameter of the driving gear. Thus, the power unit including a high-speed-rotation engine, as mounted in a motorcycle, needs an appropriate primary reduction gear ratio. However, the dimensional limitation of the driving gear and structural complexity derived from an additional shaft for supporting additional reduction gears result in a large sized power unit.
The present invention has been made to overcome the above discussed drawbacks of the existing power unit. Accordingly it is an object of the present invention to provide a power unit for a vehicle, which is capable of transmitting rotary power from a crankshaft to a pair of main shafts with fewer components and a simple structure without upsizing.